Preparation Of Hemin-Based Iron-Carbon Materials By Hydrotalcite Template Domain-Limited Method And Oxygen Reduction Performance Study

Since the high cost of platinum-based noble metal catalysts has hindered the industrial development of energy conversion systems,researchers have spent a lot of effort in the last decade to find alternatives to noble metals.Among the many solutions,transition metal-nitrogen doped carbon materials have shown catalytic activity comparable to that of platinum-based noble metals,especially single-atom dispersed Fe-N-C catalysts and heteroatom-doped catalysts.Hemin（Hm）,a natural porphyrin from a wide range of sources,is a high-quality Fe-N-C catalyst precursor.And the iron atom in the center of the porphyrin can not only form Fe-N₄coordination structure in the plane,but its axial direction can also combine ligands containing N and S,which makes the heteroatom more convenient to hybridize into the carbon skeleton.In this study,three highly active ORR catalysts were prepared using hemin as the catalyst precursor and exploiting the domain-limiting effect of hydrotalcite（LDH）.A series of characterization tests were used to analyze the composition and structure of the catalysts,and their catalytic activities were investigated by electrochemical tests.（1）By using the recombination effect of hydrotalcite to confine hemin in the layer spacing,hemin was polymerized along the hydrotalcite lamellae during the carbonization process,resulting in a graphene-like structure of the two-dimensional sheet-like carbon material Hm-LDH-700.This unique two-dimensional structure gives the carbon material a large specific surface area(1065.79 m²g^-1)and a rich pore structure.Synchrotron radiation tests show that single-atom dispersed Fe-N₄active sites exist in the carbon material,the presence of Fe-N_xactive sites can promote the ORR reaction process because it reduces the adsorption energy of O₂and stretches the O-O bond simultaneously.Electrochemical tests show that Hm-LDH-700has a half-wave potential of 0.857 V vs.RHE and a Tafel slope of only 74 m V dec^-1,which is much better than commercial Pt/C.Hm-LDH-700 also exhibits excellent methanol tolerance and long-term stability.（2）Inspired by the coordination of hemin and histidine（His）in human hemoglobin,using the coordination interaction between hemin and histidine,then confine them between hydrotalcite laminate to prepare a highly active ORR catalyst modified by histidine.Theoretical calculations show that histidine formed coordination bonds with Fe atoms in hemin through N atoms in the imidazole ring.X-ray photoelectron spectroscopy（XPS）confirms the enhancement of the total nitrogen content in the material（10.89%）,the increase of defects in the carbon skeleton due to the successful doping of N atoms,the abundance of defect sites and the increase of the total nitrogen content can promote the O₂molecule adsorption at the active sites and be helpful to stretch O-O bond.Electrochemical tests show that Hm-LDH-His-700 has a half-wave potential of 0.904 V vs.RHE and a Tafel slope of 59 m V dec^-1,which is significantly superior compared to commercial Pt/C.In addition,Hm-LDH-His-700 exhibits excellent electrocatalytic stability and methanol resistance.（3）Inspired by the Fe-S axial coordination bond in human cytochrome C,a sulfur-doped Fe-N-C two-dimensional carbon nanomaterial was synthesized.Theoretical calculations showed that thiourea（Thi）and hemin formed coordination compounds via Fe-S coordination bonds.XPS demonstrated the successful hybridization of S in the carbon skeleton,forming C-S-C and Fe-S active sites.The doping of large-size S caused an abundance of defect sites（I_D/I_G=1.2）and charge redistribution on the carbon skeleton,which resulted in easier O₂adsorption and weakened O-O bond energy.As a result,Hm-LDH-Thi-700 exhibits excellent electrocatalytic activity with a half-wave potential of 0.915 V vs.RHE and a Tafel slope of only50 m V dec^-1,which has a faster reaction kinetics compared to commercial Pt/C(E_1/2=0.882 V vs.RHE,91m V dec^-1).It also performs well in terms of methanol resistance and stability.